Reverse osmosis is most commonly used to treat water containing dissolved salts. Treatment is carried out by flowing a water stream across the feed side of a membrane. Pressure in excess of the osmotic pressure of the feed solution is applied on the feed side, and under this pressure driving force, water molecules pass through the membrane preferentially. The treated water, containing a much lower concentration of salt than the feed water, is withdrawn from the permeate side. Dissolved salts, organic compounds, colloids, microorganisms and any other matter suspended in the water are retained on the feed side.
Reverse osmosis membranes may be made by a number of membrane preparation techniques. The first commercial reverse osmosis membranes were asymmetric cellulose acetate membranes made by the Loeb-Sourirajan phase separation or phase inversion process. These membranes have lower flux and rejection than other high-performance modem membranes, but have maintained a fraction of the market because they are easy to make, mechanically tough, and relatively resistant to degradation by chlorine and other chemicals.
However, almost all reverse osmosis membranes are now made by interfacial polymerization. In this method, an aqueous solution of a reactive monomer, such as a diamine, is deposited in the pores of a microporous support membrane, typically a polysulfone ultrafiltration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, such as a triacid chloride in hexane. The amine and acid chloride react at the interface of the two immiscible solutions to form a densely crosslinked, extremely thin membrane layer. Interfacially polymerized RO composite membranes typically contain either an anionic or a cationic charge.
Current commercial RO membranes made by interfacial polymerization have sodium chloride rejections of 99+% and water fluxes greater than 35 L/m2·h at a feed pressure of 800 psig. Recent studies have shown that these membranes also exhibit rejections of 99+% for pesticides and organic micropollutants, such as chlorophenols.
The best RO membranes for seawater desalination are highly crosslinked aromatic polyamide thin-film composite membranes made by the interfacial polymerization process. These composite membranes consist of three layers: (i) a microporous support; (ii) an ultrathin selective polymer layer; and (iii) a thin, porous surface layer.
About half of the reverse osmosis systems currently installed are used to desalinate brackish water or seawater. Another 40% produce ultrapure water for the electronics, pharmaceutical, and power generation industries. The remainder are used in small niche applications such as pollution control and food processing. One reason that reverse osmosis is not applied more widely, such as to treat industrial wastewater, is the propensity of the membranes, especially those made by interfacial polymerization, to foul.
Fouling occurs when contaminants such as charged solutes, oils, bacteria, colloidal materials of various types, and suspended particulates become trapped on the surface or in the pores of the membrane. Membrane fouling is an issue for all reverse osmosis operations, and systems are usually designed to include one or more pretreatment steps upstream of the reverse osmosis units. These treatments frequently involve combinations of physical processes, such as filtration, to remove particulates, bacteria and oils, and chemical treatments to sterilize the feed water, control pH and the like.
Despite careful pretreatment, regular cleaning is also required in many cases to handle fouling that still occurs.
Typical surface structures of aromatic polyamide RO membranes made by interfacial polymerization are shown in FIGS. 4 and 6. The surface of the membrane is surprisingly rough and porous. The membrane has a “ridge-and-valley” structure with a surface pore size in the range of 0.1-0.5 micron. This membrane fouls very easily as solutes and particulates pass through the surface pores into the internal membrane structure. Additional fouling results from the interaction of the charged membrane material with charged colloids and surfactant.
Attempts have been made to modify RO membranes in different ways to improve their properties.
U.S. Pat. No. 5,989,426 discloses applying a positively-charged coating to an otherwise negatively charged polyamide membrane to improve rejection of cations by the membrane.
U.S. Pat. No. 6,026,968 discloses reverse osmosis membranes to which a hydrophilic coating containing a sulfonic acid group is added to improve anion rejection as well as chlorine resistance.
U.S. Pat. No. 6,177,011 describes reverse osmosis membranes having electrically neutral coatings intended to reduce fouling of the surface by charged particles.
U.S. Pat. No. 6,413,425 describes reverse osmosis membranes similar to those of U.S. Pat. No. 6,177,011 above, but specifically having a polyvinyl alcohol (PVA) coating. The coating reduces the surface roughness in a quantified manner.
U.S. Pat. No. 5,698,105 also describes reverse osmosis membranes with a PVA coating. The coating improves the salt rejection of the membrane.
U.S. Published Patent Application 2003/0121844 describes reverse osmosis membranes having a coating of a crosslinked epoxy compound. The coating was found to reduce fouling by dried milk or surfactant.
Polyamide-polyether block copolymers have been reported to be useful as selective layers in gas separation and ultrafiltration membranes, as in U.S. Pat. No. 4,963,165; German patent number DE 4237604; an article by K. Ebert et al., “Solvent resistant nanofiltration membranes in edible oil processing,” (Membrane Technology, No. 107, p. 5-8, 1999); and an article by S. Nunes et al., “Dense hydrophilic composite membranes for ultrafiltration,” (J. Membrane Science, Vol. 106, p. 49-56, 1995).
Fouling continues to be a serious problem for reverse osmosis membranes, and one that hampers the use of reverse osmosis except on feed streams that are very clean or have been made so by rigorous pretreatment.
There remains a need for intrinsically less fouling reverse osmosis membranes. If such a need could be filled, wider applications of reverse osmosis treatment, such as to industrial wastewaters of many types, or for military or naval use, would be possible.